1. Field of the Invention
This invention relates to rubber compositions and has particular reference to rubber compositions suitable for use in the manufacture of steel-belted automobile tires.
2. Prior Art
Steel-belted tires are in widespread use for their inherent advantages of good driving stability, high wear resistance and other superior physical properties compared to conventional tires. Research, efforts have been focused, inter alia, on improving the of adhesion between rubber layers and steel cords which has an important bearing upon the quality of the tires. This adhesion parameter determines the service life of the tire exposed to various environmental conditions during and subsequent to its processing stage and shipment to the end user. In the manufacture of a steel-belted tire, the starting rubber possesses sufficient "initial adhesion" with steel cords at the time of vulcanization. Unvulcanized rubber, however, is susceptible to absorption of moisture and hence its adhesive property, namely "unvulcanized moisture resistant adhesion", tends to decline upon vulcanization. Vulcanized rubber is also liable to suffer a reduction in its adhesive property, namely "vulcanized moisture resistant adhesion", due to penetration of moisture in the air through the rubber layer into the steel cords during the use of tire. It is further possible that if the tire receives cuts, nail punctures and other damage to its tread portion during running of the automobile, which will allow permeation of water therethrough, this water is warmed up by the heat generated in the running tire and flows through the interstices of the steel cords, causing the rubber to separate due to a loss of its adhesive property; namely "vulcanized warm water resistant adhesion".
The terms quoted above represent all aspects of the adhesive property required of rubber compositions for steel-belted tires.
Various tire rubber composition are known which are capable of meeting some, but not all of the above adhesion requirements.
The vulcanization process in the manufacture of steel-belted tires holds the key to productivity as a whole. Vulcanization time can be shortened by increasing its temperature. However, conventional rubber compositions containing cobalt salts of organic acids, though believed conducive to adhesion with steel cord, are liable to suffer a decline in their "unvulcanized moisture resistant adhesion", "vulcanized moisture resistant adhesion" and "vulcanized warm water resistant adhesion" because of the higher vulcanization temperature.
It is known to choose high hardness and high modulus rubbers to cover hard steel cords for steel-belted tires. Such hard rubbers often incorporate large quantities of sulfur and carbon black and exhibit increased hardness and modulus upon vulcanization. At the same time, however, they tend to deteriorate in tensile strength and breaking extension.